1. Field of the Invention
The invention relates to an electronic control circuit for use as a safety device for controlling the motion produced by a drive motor which is employed to raise and lower a sliding window and, in particular, an automotive window.
2. Description of the Prior Art
A safety detector, i.e., proximity detector, for use in an automotive electrically operated sliding window, as described, for example, in U.S. Pat. No. 3,651,389, comprises a conductor used as a capacitive pickup and oriented along the upper edge of the window. Capacitance of the pickup varies whenever a portion of a human body, e.g., hand, approaches it. The pickup is connected to an oscillator which, in turn, is connected to a detector contained within a control circuit. As the capacitance of the pickup varies, the amplitude of the signal produced by the oscillator and appearing across the pickup also varies. This detector is comprised of an amplifier, which in response to changes in the amplitude of the signal appearing across the pickup, causes the power applied to a motor, which raises and lowers the window, to be cut off.
Such a safety detector should prevent the window from moving when the pickup is in direct contact with a portion of a human body, e.g., a hand. However, to be totally effective, the detector must always prevent the window from moving whenever a nonconductive contact occurs; for example, when a gloved hand touches the window. The detector should, therefore, respond to a small change in the capacitance of the pickup and, therefore, must have a relatively high sensitivity. However, whenever the detector is set to such a high sensitivity, the control circuit often reacts to certain spurious influences, i.e., changes imparted to the signal which are totally unrelated to the approach of a part of the body to the capacitive pickup.
These changes occur, for example, when a moisture deposit appears on the glass, either in the form of mist or water film, or even as the result of a salt deposit. In addition, spurious influences also occur whenever the pickup approaches the window frame, or if the respective positions of various elements of the circuit change during movement of the window or, lastly, if these elements encounter considerable temperature variations.